Determination of sodium hydroxide in presence of the aluminate



Patented Oct. 26, 1943 UNITED STATES PATENT OFFICE DETERMINATION OFSODIUM HYDROXIDE IN PRESENCE OF THE ALUIVHNATE Perle N. Burkard,Wyandotte, Mich., assignor, by mesne assignments, to Wyandotte ChemicalsCorporation No Drawing. Application January 6, 1941, Serial No. 373,323

8 Claims.

their conditions and circumstances of usage, have become contaminatedwith an alkali aluminate. My invention is equally well applicable tocaustic alkali solutions in which the caustic alkali is sodium orpotassium hydroxide and the corresponding contaminating compound issodium or potassium aluminate. Sodium hydroxide, however, is the causticalkali most commonly used commercially in detergent, cleaning andwashing solutions. with metallic aluminum, a chemical reaction takesplace whereby some or all of the alkali is converted into sodium (orpotassium) aluminate, thereby decreasing the alkaline intensity of thesolution and its corresponding detergent and germicidal ability.

My invention is based upon the discovery that the addition of sodiumfluoride to such solutions which have become contaminated with sodium orpotassium aluminate, results in a substantial increase in the alkalinitythereof; in fact, in tripling the alkalinity or hydroxyl ionconcentration of.

the aluminate. This discovery is primarily of a two-fold utility.

First it provides a new convenient and reliable analytical method fordetermining both the caustic alkali and aluminate content of alkalinesolutions which have become contaminated with alkali metal aluminate;and secondly, as already indicated it provides a convenient method ofregenerating the alkalinity of such solutions of deficient alkalinecapacity.

In the art of alkaline cleaning for the food and beverage industries,food containers, beer bottles, milk bottles, and numerous other articlesare washed in alkaline solutions containing sodium or potassiumhydroxide in combination with alkali metal salts, such as trisodiumphosphate,

As such solutions come in contact ingredient in bottle washingcompounds. Various other alkalies, such as the alkali metal phosphates,carbonates, bicarbonates, borates and silicates, are used in sodiumhydroxide to improve its rinsing properties and to condition the waterfor more effective cleaning.

Such alkali detergent solutions, when coming in contact with metallicaluminum, such as that found in metal foil labels, bottle caps, utensilsand the like, reacts to form alkali metal aluminate according to thefollowing equation:

The other alkali metal salts present also react with the aluminum in asimilar manner, forming additional aluminate. The presence of thealuminatein such solutions is undesirable, first, because it is a weakerand much less effective alkali, thus depreciating the alkaline capacityor hydroxyl ion concentration of the solution as a whole; and secondly,because it is impossible, with analytical testing methods heretoforeknown, reliably to determine the actual amount of alkali metal hydroxidepresent in the solution. It is, therefore, the general object and natureof my invention to provide a method for regenerating the alkalinity ofalkaline,.detergent soluof my analytical method may be accuratelydetermined in dirty and soil-contaminated solutions, To theaccomplishment of the foregoing and related ends, said invention, thenconsists of the steps hereinafter fully described and particularlypointed out in the claims.

The following description sets forth in detail one approved method ofcarrying out the invention, such disclosed method, however, constitutingbut one of the various ways in which the principle of the invention maybe used. Although, as above indicated, the invention is equivalentlyapplicable to alkali metal compound solutions in which the hydro des d cnating aluminates are those of sodium orpotasslum, the followingdetaileddescription will suitably, and for the sake of convenience, be confinedto the sodium compounds.

As previously pointed out, the contamination oi alkali, detergentsolutions with sodium aluminate has presented a serious problem in theart. The seriousness of this problem can be appreciated by consideringthat the aluminum labels from 210,000 average iii-ounce beer bottleswill completely convert all the sodium hydroxide in 600 gallons of a 3percent sodium hydroxide solution into sodium aluminate, a milder andmuch less eflective alkali. In addition, by the methods commonly usedfor titrating alkaline solutions, this solution will still show a 3percent sodium hydroxide contentwhen no sodium hydroxide is present.

For operating efllciency and to comply with certain public healthregulations, it is desirable to maintains deflnite sodium hydroxidecontent in the solution. Some State laws. for instance, require a 3percent sodium hydroxide content. To do this, a reliable method oftesting the solution must be employed. The methods used in the pasthavebeen:

(1) By determining the specific gravity of the solutions with ahydrometer and reading the sodium hydroxide content from a conversiontable.

(2) By the double titration method employing a standard acid solution,phenolphthalein indicator, and methyl orange indicator and calculatingthe sodium hydroxide content.

(3) By precipitating the other alkaline salts with neutral bariumchlorideand titrating the sodium hydroxide which remains in solutionwith a standard acid using phenolphthalein indicator solution.

Method (1) is accurate for sodium hydroxide solutions, and, since sodiumhydroxide-is seldom used alone,.this method is not accurate enough foraccepted use.

Method (2) is accurate only ior solutions con taining sodium hydroxidein combination with sodium carbonate and/or trisodium phosphate.

Method (3) has been most widely used and for general use is the mostaccurate, but it still is not sufllciently accurate for solutimscontaminated with sodium aluminate.

1'. have discovered that the alkalinity of a soduim aluminate solutionabove a pH of 8.2 can be tripled by adding an excess of sodium fluoride,preferably in the form oi. a powder. Thechemicalreactionthattakupiacecannotatthis time be fully explained.Sodium aluminate, NaAlOz, reacts inthe presence of sodium fluoride as itits iorrnula were NasAlOa. Thus, by adding a chemical equivalent amountof powdered sodium fluoride to a solution containing sodium aluminate,the spent alkalinity due to the presence oi the sodium aluminate isregenerated. Such a chemical equivalent amount of the sodium fluorideis, oicourae, estimateduponthebasisoitheprobtetrasodium pyrophosphatc,sodium hexametaphosphate, borax, sodium bi-carbonate, sodium silicateand sub-silicates. The solution is then titrated with an acid solutionof known strength using a few drops 01' 0.5 percent alcoholicphenolphthalein solution as the indicator. When the end point is reachedat a pH of approximately 8.2, only the alkalinities or the sodiumhydroxide and the sodium aluminate have been neutralized. The otheralkaline materials have reacted with the barium chloride precipitatingbarium salts and forming neutral sodium chloride. If at this point anexcess, usually about 0.5 to 1.0 gram for every 10 ml. of alkalinesolution, of sodium fluoride powder be added, the solution will againturn pink due to the alkalinity formed by the reaction of the aluminateand the sodium fluoride. I have discovered that this increase inalkalinity upon the addition of the sodium fluoride powder is equal totwice the alkalinity oi the sodium aluminate present in the originalsolution.

Applying the above-stated facts to chemical analysis, I have discoveredthat, by titrating an aliquot portion of an alkaline solution with anacid of known strength in the presence of barium chloride andphenolphthalein indicator solution, the number of milliliters of acidused in this titration minus 5 the number of milliliters of acidrequired to reach the second end point after the addition of sodiumfluoride powder is equal tothe number of milliliters of acid required toneutralize the sodium hydroxide present in the aliquot portion of thealkaline solution. If 1.000 normal acid were used, each milliliter ofacid is equivalent to 0.040 gram 01' sodium hydroxide.

For example, in titrating 10 milliliters of an alkaline solution havinga specific gravity of 1.03

to which barium chloride has been added in ex-' cess of that amountrequired to precipitate carbonates, phosphates, etc., with 1.000 normalsuli'uric acid using 0.5% alcoholic phenolphthalein as the indicatorsolution; 16.5 milliliters of the acid were required to reach the endpoint. After the addition of sodium fluoride, 10.4 milliliters ofadditional acid was required to reach the second end point. Then:

(16.5- 10.4/2) X .040X 100 "1o 1.o3 In practical fleld testing. I employ2.56 normal sulfuric acid and titrate lo-milliliter portions 01 thecleaning solution. The use or 2.58 normal acid in place of 2.50 normalacid compensates apable amount or sodium aluminate present and is addedin excess thereoiinorder to insure asate the preceding paragraph thenbecome:

1st end point. 6.44 milliliters of 2.58 8 acid 2nd end point at. theaddition or sodium fluoride---- 4.07 milliliters of 2.56 8 acid%NaOH=6.44--4.07/2=4.4

The sodium aluminate can be obtained by multiplyins the milliliters ofacid used in themond titration by a constant factor. factor is derivedas inflows:

Since all oi the acid used in the second titration is used in titratingthe alkali liberated by the re- I action oi sodium aluminate and sodiumfluoride, it means that 1 molecule of sulphuric acid is regfggX.l255=0.104ii grams N aAlO;

The above factor is converted into terms of per cent of sodium aluminateby multiplying it by 100 and dividing by 10.3, viz.

The percent of sodium aluminate in our illustrative example is thuscalculated as follows:

% NaAlOz=L019 X 4.07

In some instances, it might beyfound necessary to adjust the abovederived factor of 1.019 to compensate for the presence of otherchemicals in solution and in practical field tests. This ad- .iustmentcan be made in the factor by titrating a number of solutions of knownstrength and calculating the factor.

'The method herein described works equally well for solutions from theirfreezing to their boiling points and for dirty solutions, as well asclean solutions. To overcome the necessity of filtering dirty solutions,I have purposely used barium chloride and sulfuric acid to form aninternal, white background oi white barium sulfate to make indicatorcolor changes easily visible.

It will be apparent to those skilled in the art that equivalentindicators other than the phenolphthalein indicator hereirabovementioned may be employed to indicate the pr p end point during theneutralizing or titrating steps of my analytical method.

By the 'same token, equivalent neutralizing acids, made up in standardsolutions, may likewise be employed in lieu of the standard sulfuricacid solutions hereinabove described.

The above-described method for determinin the caustic alkali and sodiumaluminate content of solutions is highly accurate and reliable forpractically all alkaline detergents encountered in the fleld, even thosecontaining silicates in the amount of up to 1% sodium silicate. Abovethat amount, silicates will have some eifect upon the accuracy of thesodium hydroxide content determinations. but any such higher percentageof sodium silicate is rarely encountered in solutions contaminated withaluminum, for the simple reason that detergents containing silicates arenot ordinarily employed if the solutim is likely to be placed in contactwith aluminum.

Other modes of applying the principle of my invention may be employedinstead of the one explalned, change being made as regards the methodherein disclosed, provided the step or steps stated by any of thefollowing claims 01' the equivalent of such stated step or steps beemployed.

1, therefore, particularly pointout and distinctly claim as myinvention:

1. The method of determining the amount of alkali metal hydroxides insolutions thereof contaminated with alkali metal aluminate, consisthisin the steps of neutralizing the alkalinity of said hydroxides and saidaluminate, adding sodium fluoride thereto in an amount suiilcient toreact completely with the aluminate, thereby regenerating the alkalinityof said aluminate to twice its original amount, and then neutralizingthe resultant solution.

2. The method of determining the amount of sodium hydroxide in solutionsthereof contaminated with sodium aluminate, consisting in the 7 steps ofneutralizing the alkalinity of said hy-' droxide and said aluminate,adding sodium fluoride thereto in an amount sufllcient to reactcompletely with the sodium aluminate, thereby regenerating thealkalinity of said aluminate to twice its original amount, neutralizingthe resultant solution, and measuring the sodium hydroxide chemicalequivalent amount of neutralizing reagent required in each of theaforesaid neutralizing steps.

3. The method of determining the amount of sodium hydroxide and sodiumaluminate present in the same solution, consisting in the steps ofneutralizing the alkalinity of said hydroxide and said aluminate, addingsodium fluoride thereto in an amount suflicient to react completely withthe sodium aluminate. thereby regenerating the alkalinity 01 saidaluminate to twice its original amount. neutralizing the resultantsolution, and measuring the sodium hydroxide chemical equivalent amountof neutralizing reagent required in each of the aforesaid neutralizingsteps.

4. The method of determining the amount of sodium hydroxide in asolution containing alkali metal salts of weak acids in additionthereto, and contaminated with sodium aluminate, consisting in the stepsof precipitating said alkali metal salts. whereby the sodium hydroxideand sodium aiuminate remain in solution, neutralizink the alkalinity ofsaid hydroxide and said aluminate, adding sodium fluoride thereto in anamount sufiicient to react completely with the sodium aluminate, therebyregenerating the alkalinity of said aluminate to twice its originalamount,- and then neutralizing the resultant solution.

5. The method of determining the amount 0 sodium hydroxide in a solutioncontaining alkali metal salts of weak acids in addition thereto andcontaminated with sodium aluminate, consisting in the steps ofprecipitating said alkali metal salts, whereby the sodium hydroxide andsodium aluminate remain in solution, neutralizing .the

alkalinity of said hydroxide and said aluminate, adding sodium fluoridethereto in an amount sumcient to react completely with the sodiumaluminate, thereby regenerating the alkalinity of said aluminate totwice its original amount,

then neutraliziing the resultant solution, and measuring the sodiumhydroxide chemical equivalent amount of neutralizing reagent required ineach 'of the aforesaid neutralizing steps.

6. The method of determining the amount of sodium hydroxide and. sodiumaluminate, both present in the same solution, and in which alkali withthe sodium aluminate, thereby regeneratin the alkalinity of saidaluminate to twice its original amount, neutralizing the resultantsolution,

and measuring the sodium hydroxide chemical equivalent amount ofneutralizing agent required in each of the aforesaid neutralizing steps.

7. The method of determining the amount of sodium hydroxide in asolution containing alkali metal salts of weak acids in addition theretoand contaminated with sodium aluminate, consisting in the steps ofadding barium chloride to said solution in excess of the amount requiredto precipitate said alkali metal salts. thereby leaving the sodiumhydroxide and sodium aluminate in solution, adding an indicator to saidsolution and titrating to the neutral end point with a standard solutionof sulfuric acid, thereby forming a white background of barium sulphatein the solution, adding sodium fluoride to the solution in an amountsuiiicient to react completely with the sodium aluminate, therebyregenerating the alkalinity of said aluminate to twice its originalamount, and again. titrating to the neutral 20 normal solution ofsulfuric acid, thereby forming a white background of barium sulphate inthe solution, adding sodium fluoride to the solution in an amountsuiiicient to react completely with the sodium aluminate. therebyregenerating the alkalinity of said aluminate to twice its originalamount, and again titrating to the neutral end point with a 2.58 normalsolution of sulfuric acid and measuring the respective amounts of saidsulfuric acid solution required in both of the aforesaid titrationsteps.

PERLE N. BURKARD.

